A scintillator is a material which absorbs a radiation, such as γ-ray or X-ray, and emits an electromagnetic wave with a wavelength of visible light or near visible light. The applications thereof include various types of radiation detectors, such as medical positron emission tomography (PET), time-of-flight positron emission tomography (TOF-PET), and X-ray computed tomography (X-ray CT), and further a survey instrument for the inspection of personal belongings used at an airport and the like.
Such a radiation detector generally consists of a scintillator unit, which absorbs a radiation and converts it into visible light, and a light detector, such as a photomultiplier tube (hereinafter, referred to as “PMT”) or a photodiode, which detects the visible light converted in the scintillator unit and transmitted therethrough and converts it into an electrical signal. Thus, the scintillator used for this type of applications is desired to be a scintillator with a high output level of light emission in order to reduce noise and increase measurement accuracy.
As a scintillator, in the related art, an alkali halide crystal, such as CsI or NaI, has been widely practically used. Among them, a scintillator having CsI, as the host material, is used in terms of that the efficiency of radiation absorption thereof is relatively high, the radiation damage thereof is relatively small, and it is relatively easy to manufacture a thin film thereof by a vacuum deposition and the like.
However, in a CsI scintillator of the related art, light emission efficiency is not so high and the decay time of the fast component of fluorescence is not sufficiently short, and thus scintillators, such as CsI:Na and CsI:Tl, which are doped with thallium iodide (TlI), in which the efficiency of scintillation is increased by doping a crystal having CsI, as the host material, with impurities are practically used. In JP 2008-215951A, for example, cesium iodide:thallium (CsI:Tl), in which cesium iodide (CsI) is doped with thallium (Tl), is disclosed.
When cesium iodide:thallium (CsI:Tl), in which CsI is a host material and doped with thallium, is used as a scintillator, it is possible to obtain a high output level by combining with a photodiode, as a detector, but there is also a problem that an afterglow, which tends to obscure the image, is caused. Especially, when an instrument for hand baggage inspection or the like images a moving object, the afterglow property is important, and hence this problem needs to be solved.
Thus, the invention is to provide a novel scintillator, in which the afterglow property of cesium iodide:thallium (CsI:Tl), in which CsI is the host material and doped with thallium, is improved.